The Cassegrain unit, inspired from ESPaDOnS spectropolarimeter, consists of 2 main modules mounted at the Cassegrain focus of the telescope.
The upper Cassegrain module serves as a mechanical interface to the telescope and provides the usual spectropolarimetric calibration facilities - in particular:
- an Atmospheric Dispersion Corrector correcting the entrance beam for the refracting effect of the atmosphere,
- a tip-tilt module stabilizing the entrance image
- a wheel allowing to inject light from the various calibration lamps used for spectroscopic observations
The lower Cassegrain module mainly consists of an achromatic polarimeter, including tunable polarization optics that splits the incoming beam into 2 separate images, each injected into a separate fiber at polarimeter output. By tuning the polarimeter optics, the polarimeter can be set to measuring either circular or linear polarization.